chore: remove dead plasma-theme pkg, fix optdepends and dead cmake flag

- drop antergos-plasma-theme: superseded by antergos-layan-theme, the
  active look-and-feel (com.github.vinceliuice.Layan). Package had no
  dependents, was not referenced by any calamares config or netinstall
  group, and pulled plasma-desktop as a hard dep. Removed from
  packages.yaml, README, and deleted the package directory. The
  start-here-kde-antergos.svg it shipped is a self-contained copy inside
  antergos-layan-theme, so nothing breaks.
- antergos-next-desktop-settings pkgrel 1-4: add descriptions to
  optdepends (qt5ct/qt6ct) per Arch packaging guidelines (closes #2).

Calamares pkgrel 3.4.2-10:
- remove -DCMAKE_DISTRIBUTION_NAME: calamares ignores it; the 'for
  Antergos NeXT' about text comes from versionedName in the active
  branding component at runtime (closes #3).

packages/calamares/PKGBUILD

@@ -5,7 +5,7 @@
 _pkgname="calamares"
 pkgname="$_pkgname"
 pkgver=3.4.2
-pkgrel=9
+pkgrel=10
 pkgdesc="Distribution-independent installer framework"
 url="https://codeberg.org/Calamares/calamares"
 license=("GPL-3.0-or-later")
@@ -82,7 +82,6 @@ build() {
     -DKDE_INSTALL_MANDIR=/usr/share/man
     -DKDE_INSTALL_INFODIR=/usr/share/info
     -DKDE_INSTALL_SYSCONFDIR=/etc
-    -DCMAKE_DISTRIBUTION_NAME="Antergos NeXT"
     -Wno-dev
   )